1. Field of the Invention
The present invention relates to communications networks. More particularly, the invention relates to an improved design for an optical fiber communications network link using low-power optical line amplifiers which obviates the need for regenerators.
2. Related Art
A typical communications network, serving to transport information between a number of locations, includes various physical sites, called nodes, interconnected by information conduits, called "links." Each telecommunications link serves to carry information from one site to another. Individual sites contain equipment for combining, separating, transforming, conditioning, and/or routing data.
Such telecommunications links are generally implemented using electrical cables, satellites, radio or microwave signals, or optical connections. Telecommunications links can stretch for tens or hundreds of miles between sites. Through these links, a telecommunications system can carry voice and data signals between and among nodes to effectively interconnect data remote equipments, e.g., computers, remote terminals, servers, etc.
Optical fiber telecommunications networks include a plurality of optical fiber transmission lines, also known as "optical fiber links". Such optical fiber links facilitate high bandwidth communications and can be used in voice and data network systems. High speed data can be modulated on light waves which are transmitted through the optical network. The optical transmission line, connecting an optical transmitter and receiver, can propagate many light wave signals of different frequencies simultaneously. Thus, fiber optic communications links carry vast amounts of information among distant sites to facilitate data and voice connectivity over a large geographical area.
Through these optical fiber links information is transported from node to node in the form of an optical signal. Electrical data-carrying signals are manipulated within the nodes. The electrical data-carrying signals are usually not suitable to be directly propagated along the optical fiber links. Instead, a Line Terminal Equipment (LTE) at either end of the optical fiber link serves to convert the data-carrying signals between the optical and electrical domains and to amplify the data-carrying signals. By converting and amplifying the data-carrying signals, the signals can traverse a long link and can be faithfully rendered at a distant node. In this approach, an LTE at one node converts data-carrying electrical signals into near-infrared optical signals that are then coupled into a long, slender optical fiber. At the other end of the optical fiber link, an optical receiver, as part of a remote LTE, detects the optical signals and recreates the corresponding data-carrying electrical signal.
Frequently, optical fiber links are so long that several intermediate amplification stages are required along the length of the fiber. Amplification can be accomplished by using optical line amplifiers. The most common optical amplifiers include Bi-directional Line Amplifiers (BDLAs) which incorporate high gain amplifiers, either erbium-doped fiber amplifiers (EDFAs), which amplify with a laser pump diode and a section of erbium-doped fiber, or semiconductor laser amplifiers.
Performance of the high-bandwidth optical fiber link can be impaired by several inherent properties of optical fiber. One property, fiber loss, reduces the average power reaching the receiver of the LTE over the optical fiber link. Fiber loss is the attenuation of the optical signal over the link. Optical receivers require a certain minimum amount of power in order for the LTE to accurately recover the signal and the information contained in the signal. The optical line amplifiers used in the intermediate amplification stages can increase the average power reaching the receiver, thus compensating for fiber loss. Thus, loss compensation can be carried out by optical amplifiers which amplify the optical bit stream directly without requiring conversion back into the electrical domain. Two peculiar aspects of optical line amplifiers are that stronger incoming signals receive more amplification than weak signals and that the frequency response is noticeably uneven across the popular erbium band. Thus, in a group of wavelengths traveling through the link, one wavelength may successively gain in strength enough to disadvantage other wavelengths.
In addition to using optical line amplifiers to compensate for fiber loss, very long optical links require one or more optical regenerators to be placed at intervals along the fiber link which convert the optical bit stream back into the electrical domain. A detailed description of fiber-optic communication systems is described in Fiber-Optic Communication Systems, by Govind P. Agrawal, page 186, the text of which is incorporated herein by reference, in its entirety. It is commonly thought that when the link length exceeds a certain value, in the range of 20-100 km depending on the operating wavelength, it becomes necessary to compensate for fiber loss using a regenerator, as the signal would otherwise become too weak to be detected reliably. A regenerator is a type of repeater or "re-modulator" which performs additional functions. A re-modulator includes a receiver (demodulator), a pulse recovery, retiming, and reshaping ("3R") device, and a retransmitter (modulator) connected in succession. When the regenerator receives a degraded optical signal, it first converts the signal into the electrical domain. Then the regenerator recovers the clock or electrical bit stream from the degraded optical signal, retimes and reshapes the signal pulses. The regenerator converts the signal back into an optical bit stream by modulating the transmitter. The regenerator finally retransmits the fresh representation of the signal along the optical fiber.
In addition to loss, the fiber medium can introduce other impairments to fiber optic performance, such as dispersion. Dispersion broadens optical pulses as they propagate in the fiber, because different spectral components of the pulse travel at slightly different group velocities. The group velocity associated with the fundamental mode is frequency dependent because of chromatic dispersion. This phenomenon is referred to as group-velocity dispersion (GVD), intramodal dispersion, or simply fiber dispersion. Fiber dispersion includes material dispersion and waveguide dispersion contributions. Waveguide dispersion is no longer a problem with the advent of single mode fibers.
Optical amplifiers have attracted considerable attention recently for overcoming fiber loss. Long-haul lightwave systems are now thought to be limited by fiber dispersion rather than by fiber loss. For such long-haul systems, optical amplifiers cannot be cascaded indefinitely, since dispersion-induced pulse distortion eventually limits the fiber optic system performance. A signal that goes through a link incorporating regenerators does not suffer from this dispersion problem are less likely to suffer from dispersion, because regenerators correct the pulse broadening effect by recovering the clock signal, retiming by reapplying the clock, and reshaping the signal. Thus for long point-to-point links, regenerators are thought to be the preferred means of loss compensation.
The spacing between repeaters, i.e. between two optical line amplifiers or an optical line amplifier and a regenerator, known as the transmission distance spacing, is traditionally an important design parameter, since system cost is thought to be reduced as the spacing between repeaters increases. However, transmission distance spacing depends on the bit rate because of fiber dispersion. The product of bit rate and the transmission distance spacing is generally used as a measure of system performance for point-to-point links. This product depends upon the operating wavelength, since both fiber loss and fiber dispersion depend upon wavelength. Thus, the traditional goal in designing a point-to-point link is to maximize the transmission distance spacing between repeaters so as to minimize overall system cost.
Modern optical fiber communications networks spanning long optical links are thought to require the use of regenerators. A regenerator is a relatively complex and expensive facility. Because a regenerator performs optical retransmission it must be equipped with receivers and transmitters of the correct wavelengths. Because of the electrical domain functions that are performed by a regenerator, a regenerator is also limited in the range of signal types that it can handle. This inflexibility under certain circumstances could present problems in the field. In performing network upgrades or circumventing failures, a given link may need to readily service a variety of carrier wavelengths and modulation formats. Presently, an extensive retrofit of a regenerator would be required to accommodate a change in link traffic. Modern communications networks are thought to be incapable of maintaining signal quality over long distances without the use of regenerators.
Wavelength division multiplexing (WDM) is a way of increasing the capacity of an optical fiber by simultaneously operating at more than one wavelength. WDM multiplexes signals by transmitting the signals at different wavelengths through the same fiber. WDM works similarly to frequency domain multiplexing (FDM). In optical communications, WDM is any technique by which two or more optical signals having different wavelengths are simultaneously transmitted in the same direction over one strand of fiber and then separated by wavelength at the distant end. Four wavelength wave division multiplexing (4WL-WDM or Quad-WDM) is a method of allowing a single fiber to accomodate four light signals instead of one, by routing them at different wavelengths through the use of narrow-band wave division multiplexing equipment. The technology allows transmission of four times the amount of traffic along an existing fiber. A backbone network's capacity which normally might operate at 2.5 Gbps over a single strand, would rise to 10 Gbps using Quad-WDM.
Thus, what is needed is an improved optical link for traversing exceptionally long distances that maintains signal quality, but also can handle a variety of carrier wavelengths and modulating schemes without requiring substantial facility changes.